Immunohistochemical examinations were performed on rat pulmonary tumors induced by inhalation exposures to 239PuO2 aerosols, or by X-ray-irradiation to identify and compare cellular origins or, in turn, target cells at risk for radiation carcinogenesis. Both plutonium-induced and X-ray-induced pulmonary tumors appeared to occur from the lower respiratory tract epithelium through bronchioles into alveoli, and were histopathologically diagnosed as adenoma, adenocarcinoma, adenosquamous carcinoma, and squamous cell carcinoma. Immunohistochemical staining of neoplastic lesions using rabbit polyclonal antibodies to rat surfactant apoprotein A specific for alveolar type II pneumocytes, and Clara cell antigen specific for nonciliated bronchiolar Clara cells, showed that most of the adenomatous and adenocarcinomatous lesions from plutonium-exposed or X-irradiated rats were positive for either or both antigens, while, in contrast, adenosquamous and squamous lesions were mostly negative for both antigens. Even though there were some differences in the proportions and distributions of immunoreactive cells between plutonium- and X-ray-induced tumors and among neoplastic lesions, the results indicate that radiation-induced pulmonary adenomas and adenocarcinomas mostly originate from either alveolar type II pneumocytes or bronchiolar Clara cells, while adenosquamous and squamous carcinomas may be derived from the other epithelial cell components, or might have lost specific antigenicity during their transforming differentiation.